In our copending applications Ser. No. 479,092, filed June 13, 1974, entitled "Deep Ocean Mining Ship" by John R. Graham and Robert C. Crooke, inventors, and Ser. No. 479,091, filed June 13, 1974, entitled "Apparatus for Raising and Lowering Pipe String From a Floating Vessel", by Robert C. Crooke and Abraham Person, inventors, both assigned to the same assignee as the present invention, there is described a marine vessel equipped to carry out subsea mining operations on the ocean floor in ocean depths of 3 to 4 miles. The mining equipment is lowered to the ocean floor from the surface vessel by means of an assembled string of pipe which is lowered section by section at substantially constant velocity by equipment supported on the deck of the vessel. Because the increasing load of the mining equipment on the string of pipe as more and more sections of pipe are added to reach these great depths, it is necessary to raise and lower the pipe at substantially constant velocity to avoid the very large forces that would be encountered in a rapid acceleration or deceleration of the pipe string. This constant velocity feed is accomplished, as described in the copending applications, by isolating the equipment that raises and lowers the pipe string from the vertical motions experienced by the floating vessel from which the mining equipment is supported.
The problem of supporting a string of pipe from a floating vessel is also encountered in carrying out subsea oil drilling operations and various schemes have heretofore been proposed for isolating the drill stem from vertical movements of the floating vessel. Such known arrangements for isolating the drill stem from the vertical movements of the supporting vessel have required some stable point of reference, such as a cable, riser, or other equipment which is anchored to the ocean floor. A servo-operated mechanism which senses vertical movements of the vessel relative to the reference adjusts the linkage between the vessel and the pipe string to provide compensating adjustment in the supporting structure.